(a) Field of the Invention
The present invention concerns a semiconductor device and its manufacturing method, and more particularly it pertains to pn junction semiconductor devices using Group II-VI compound semiconductors and their manufacturing method.
(b) Description of the Prior Art
Researches and developments of pn-junction light-emitting diodes which are so-called LED using various kinds of semiconductor materials are under way at present.
There have been manufactured and are being manufactured various different kinds of light-emitting devices having different light-emission wavelengths to comply with various different purposes. These wavelengths roughly range from infrared region to green color in the visible region. Some of their examples are as follows.
LEDs made of GaAs having an energy band gap of about 1.43 eV and doped with Zinc (Zn) emit light in the infrared region, whose peak intensity is about 9100 .ANG.. LEDs made of GaP having an energy band gap of 2.26 eV doped with Zinc (Zn) and oxygen (O) emit light of red color of visible range, whose peak intensity is about 7000 .ANG.. In case it is doped with nitrogen (N), the LED emits light of green color having its peak intensity at about 5500 .ANG. or light of yellow color having its peak intensity at about 5800 .ANG.. Also, in mixed crystals GaAl.sub.1-x P.sub.x of GaP and AlP or in mixed crystals GaAl.sub.1-x As.sub.x of GaAs and AlAs, it is known that, by varying the component ratio x or by varying the doping impurity, there are obtained LEDs emitting red color light having its peak intensity of about 6500 .ANG. or yellow color light having its peak intensity of about 5900 .ANG..
The luminescence efficiency of these known LEDs is in the order of 0.01-5.00%, though varies depending on the materials employed.
The wavelength for the peak intensity of light which is emitted from an LED depends stronghly on the energy band gap of the semiconductor material with which the LED is made, and this wavelength can vary depending on the manner in which pn junction is formed.
Theoretically speaking, the wavelength for light emission at peak intensity appears in a wavelength region longer (i.e. a wavelength having a smaller energy than E.sub.g of the semiconductor material) than the absorption edge of light (that is, the absorption edge agrees with the wavelength of light corresponding to the energy band gap E.sub.g of the semiconductor material) for the semiconductor material employed. The wavelength .lambda. of the emission at peak intensity will become .lambda..sub.m which is expressed by: ##EQU1## or a greater wavelength having a lower energy than said .lambda..sub.m,
wherein:
h represents Planck's constant; PA1 c represents velocity of light; and PA1 E.sub.g represents energy band gap of the PA1 semiconductor material employed.
As stated above, the wavelengths of the lights emitting from those LEDs which have been developed and put to practice in the past are, as discussed above, cover the range of color from the infrared region inclusive to green color of the visible region. In other words, the wavelengths of emitting lights of known LEDs extend up to about 5500 .ANG. which is green color, and there has been provided no LEDs whose emitting lights have a higher energy than that mentioned above, i.e. a light of a short wavelength which, in term of color, is in the region of blue-gree, blue and violet. From the purposes of developing color electroluminescing devices and also of expanding the field of application of color LEDs, there is the demand for practicing such LEDs as having an emission wavelength of green color and such LEDs as emitting light of a wavelength shorter than that. Until now, however, no such LEDs have been obtained in the present state of technology.
As discussed above, in order to obtain an LED having such region of wavelength of emitting light, it is necessary to use semiconductor materials having an energy band gap broader than that of Group III-V compound semiconductors such as GaAs, GaP or GaAl.sub.1-x As.sub.x. As such semiconductors, there are, for example, Group II-VI compound semiconductors such as ZnSe (E.sub.g .apprxeq.2.8 eV), SiC (E.sub.g varies widely, one of which is E.sub.g .apprxeq.3.3 eV), or GaN (E.sub.g .apprxeq.3.4 eV). These semiconductors have broad energy band gaps, so that they have attracted the interest of researchers and have been studied. Nevertheless, owing to various technical problems, they have not been put to practice yet. Moreover, in a semiconductor material having a broad energy band gap, not only it is possible to obtain emission of light in the short wavelength region, but also to cause emission of light also in the long wavelength region by varying the recombination process of carriers. Especially, the Group II-VI compound semiconductors such as ZnSe stated above have broad energy band gap, and from a considerably long time ago, crystals have been produced either as photo-conductive semiconductor or as electroluminescent semiconductor. Their basic data have been accumlated in a large number and kind. As is well known, however, there has been technical difficulty in achieving free control of the conductivity types of Group II-VI compound semiconductors. In Table 1 are shown the conductivity type and energy band gap of Group II-VI compound semiconductors which have been obtained in the past not for the aspect of conductivity control but as a natural trend of development.
TABLE 1 ______________________________________ ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Conduc- n n p n n n tivity type p Energy 3.6 eV 2.8 eV 2.2 eV 2.5 eV 1.74 eV 1.5 eV band gap ______________________________________
The semiconductor materials listed in Table 1 invariably have a broad energy band gap, and will bring about a very effective result if LEDs are manufactured with them. As stated above, however, they do not permit free control of their conductivity type. For example, ZnS, CdS or ZnSe permit one to easily obtain a conductivity type of n type. However, even by doping an acceptor impurity in order to obtain p type, the result would be that the doped materials still remain to be n type or they could become crystals of either n type or p type having a very high resistivity. Even when the material happens to become p type, its control is not possible such that the material is not in such state as allowing the formation of pn junction necessary for an LED.